1. Field of the Invention
This invention relates to apparatus and a method utilizing acousto-optic tunable filters for detecting the presence of an optical radiation source and for identifying that source, and has particular application in a threat warning system.
2. Background Information
An important problem for military defense is the need to distinguish re-entry vehicle warheads from decoys and space debris, or enemy aircraft from friendly aircraft (or missiles), or enemy ground vehicles from the background. The problem is made difficult by the short time normally available form making the distinction, by the small size of the targets, or threats, and often by large amounts of clutter. It is made even more difficult by the need to detect the threat at as large a distance as possible and to identify it as quickly as possible after detection.
In conventional optical receivers, an image is recorded at a small number of discrete wavelengths which are fixed, i.e., cannot be changed during receiver usage. These receivers either record these wavelengths simultaneously on separate detectors or record them sequentially at a relatively slow rate. Furthermore, no polarization information on the image is obtained, and the same detectors are used for both target detection and target identification. The latter limitation means that the system cannot be optimized for both target detection and target identification.
Commonly owned U.S. patent application Ser. No. 07/597,457 filed on Oct. 12, 1990 suggests the use of an acousto-optic tunable filter (AOTF) for detecting and analyzing a source of optical radiation. AOTF's are crystals which utilize acoustic power to manipulate an optical beam. For a given acoustical frequency, only one wavelength of the incident light is diffracted; all other wavelengths pass through undiffracted. The referenced application utilizes a birefringent AOTF to diffract from the incident optical radiation orthogonally polarized ordinary and extraordinary diffracted beams having a narrow bandwidth determined by the frequency of the applied radio frequency acoustic signal. The diffracted ordinary and extraordinary polarized beams are directed to separate detector arrays for spectral and polarization analysis. The bulk of the incident radiation which is undiffracted is directed to another detector array which is used for broadband detection and possibly recognition of the source. The outputs of the detectors for the diffracted ordinary and extraordinary polarized beams may be subtracted to determine the polarization of the incident light or may be added for maximum signal strength at the selected wavelength.
While the apparatus described in the referenced application has many advantages, such as, the ability to perform spectral, polarization, and broad beam analysis simultaneously, and the agility to perform the spectral analysis in real time, it is an object of the present invention to provide improved apparatus for detecting and identifying a source of radiation.
It is also an object of the present invention to provide such apparatus with a greatly improved signal to noise ratio.
It is another object to provide such improved apparatus which permits spectral analysis at two frequencies simultaneously.
It is yet another object of the invention to provide such an apparatus which permits polarization analysis using a single detector.
It is an additional object of the invention to provide such improved apparatus which has improved capability for long range detection of the radiation source.